Superalloy powders are typically produced by inert atomization processes such as argon atomization, vacuum atomization, rotating electrode process and rotary disk atomization. Water atomization processes are usually unacceptable due to the formation of a heavy surface oxide produced by a chemical reaction of the form: xMe+YH.sub.2 O=Me.sub.x O.sub.y +yH.sub.2. Reactive elements (Si, Al, Ti, Cr, Mn) are oxidized and are difficult to reduce in subsequent processing. Since oxides are detrimental to the product's mechanical properties inert atomization processes (oxygen&lt;200 ppm) are used.
Unfortunately, inert atomization processes produce spherical powders which are not satisfactory for standard die compaction processes. These powders require special consolidation practices such as HIP, Cercon, CAP, etc. which are rather expensive. Due to costs of gas atomization and consolidation, the use of powder metallurgy for superalloy production has been limited to aerospace applications where the expense is justified.
There is a need for a superalloy powder that can be die compacted using existing technology. Such a powder should have an irregular shape, small average particle size and relatively low oxygen content (about 200 ppm). Water atomization can produce the irregular powder, but the oxygen content is too large. If the oxides can be removed in a cost effective process, these powders would be commercially attractive. In the steel industry, some strides are being made to satisfy these requirements. Stainless steel powders (304L, 316L, 410 and 430 grades) containing Cr and/or Mn are available and are being used to lower the cost and improve the hardenability of the finished product. These powders are produced by water atomization under conditions that minimize the oxygen level (oxygen&lt;1550 ppm). Some of these parameters are an inert purge of the atomization chamber, lower silicon heats, use of soft water (low calcium), and minimizing liquid turbulence during melting to reduce slag impurities. Further, during processing a high temperature sintering operation is used with careful control of dew point and carbon reduction to remove any oxides. In another related process (QMP), tool steels are made from water atomized powders by producing a high carbon heat. During the sintering operation a self-generated CO-CO.sub.2 atmosphere reduces the oxygen content.
In particular, the P/M slurry method is a process whereby a water soluble binder is mixed with a water atomized metal powder, lubricants and modifiers to a clay-like consistency. It is subsequently extruded or injected molded to some shape and allowed to dry so it can be handled. The product is sintered and consolidated (i.e., HIP, Cercon, hot or cold forming, etc.) with the result being near fully dense product. This method is also amenable to injection molding (U.S. Pat. No. 4,113,480) as well as die compaction (U.S. Pat. Nos. 3,988,524 and 4,129,444).
The P/M slurry method has been examined by other researchers. Firstly, in a study by Aeroprojects under contract by the U.S. Department of Interior (14-30-2567), it was determined that slurry extrusion of elemental powders (copper and nickel) was a feasible production method for fabrication of heat exchanger tubing. U.S. Pat. No. 4,113,480 deals with the production of powder parts by injection molding of inert gas atomized, very fine (10 micron) powder. As far as is known, no work has been accomplished on the use of water atomized powders due to the tenacious surface oxides.